1. Field of the Invention
The present invention relates to an apparatus and method for S locating electrically active sites within a heart or other organ or internal body structure of an animal. In particular the present invention relates to an apparatus and method for locating originating sites of cardiac arrhythmias in humans.
2. Description of the Prior Art
One of the most common type of cardiac arrhythmia is 10 ventricular tachycardia (VT) which is typified by very rapid and ineffectual contractions of the heart muscle. In a majority of patients VT originates from a small (1-2 mm) lesion that is located close to the inner surface of the heart chamber. A known treatment of VT is mapping the electrical pathways of the heart, for example by recording arrival times of electrical pulses at numerous specific locations within the heart, and building up an isochronal activation map which may then be used to locate the lesion, i.e. the "active site", from which the mapped electrical signals originate. Once located, the site is physically ablated using an ablation catheter.
One known method and apparatus for treating cardiac arrhythmias is disclosed in U.S. Pat. No. 5,391,199 (Biosense Inc.) and has means for obtaining and displaying a perspective image of the organ to be mapped, a mapping catheter having a single mapping electrode and means to locate the distal tip of the catheter by triangulation, using non-ionizing radiation. The method involves repeatedly locating the distal tip of the catheter (and hence the site of the mapping electrode), displaying the local information from the electrode is at the correct location on the perspective image and advancing the electrode to a new site. After several repetitions an activation map is created from which the active site may be deduced. An ablation catheter is advanced to this site which is then ablated. A disadvantage with this apparatus is that the mapping catheter must be repositioned several times in order to construct the map. This is time consuming and may lead to errors since the local information usually has to be collected at the same point in the cardiac cycle each time.
A further known method and apparatus for cardiac mapping is described in U.S. Pat. No. 5,465,717 (Cardiac Pathways Corporation) in which a mapping catheter is provided having a basket assembly with a number of circumferentially spaced apart, outwardly bowed, arms, on each of which is mounted a number of mapping electrodes. This enables the isochronal activation map to be rapidly constructed, in substantially one deployment of the mapping catheter and at the identical point in the cardiac cycle.
A problem with such mapping techniques is that the location of the active site can only be inferred from the map so constructed, in the same way the location of a mountain peak may be inferred from a normal isometric contour map.
This problem is mitigated in a system for locating the 25 position of an electrically active site in the heart which is described in U.S. Pat. No. 5,433,198 (Desai). The system disclosed therein has a surface ECG monitor; a cluster of electrical activity sensors for detecting the arrival of an electrical signal from a cardiac site of interest and means for estimating the location of the cardiac site by computing displacement vector. The length of this vector depends on the arrival time of the signal at each of the sensors and on a "difference" time which is an estimation of the time difference between the detection of the signal by an electrode at the point of origin of the signal and the detection of the same signal by the surface ECG monitor. Because this difference time is only an estimation, the length of the displacement vector (and hence the location of the cardiac site) is also only an estimation. As disclosed in this patent this difference time may be between typically -44 and -40 msec, the minus sign indicating that the signal is detected by the surface ECG monitor after its detection by the sensor. Using the embodiment described, this could give an error in the location of the cardiac site of .+-.2.5 mm, which is of the order of magnitude of the size of the site itself.